The present invention relates to an overcurrent protection circuit for preventing overcurrent by limiting output current to be less than or equal to a predetermined value.
In many cases, a conventional constant voltage circuit includes an overcurrent protection circuit just in case the output terminal of the constant voltage circuit short-circuits. When the output current from the output terminal becomes greater than or equal to a predetermined value, an overcurrent protection circuit of one type interrupts the output, an overcurrent protection circuit of another type locks the output to prevent the output current from increasing anymore, and an overcurrent protection circuit of a further type gradually reduces the output voltage and the output current. For the circuit that interrupts the output, an operation to recover the interrupted output is required. For the circuit that permits continuous flow of a peak current, its output transistor is adversely affected when the peak current, at which the overcurrent protection operation is started, is set at a large value. Further, the output is not sufficiently large when the peak current is set at a small value.
The circuit that gradually reduces the output current has a foldback voltage-current characteristic, and is called a foldback current limiting circuit. In the foldback current limiting circuit, the output voltage starts decreasing and the output current also starts decreasing when the current value reaches a peak current value. The value of a short-circuit current is smaller than the peak current value when the voltage of the output terminal is zero. As a result, when the horizontal axis represents current and the vertical axis represents voltage, the voltage-current relationship shows a foldback characteristic. An overcurrent protection circuit having such a foldback voltage-current characteristic ensures protection of the output transistor and enables the current amount when the overcurrent protection operation is started to be relatively large. This enables the output to be increased.
A technique relating to a stabilizing power supply circuit, which has a foldback current-voltage characteristic and sets a small output current when a ground fault occurs in the output, has been proposed to reduce heat loss and enable the circuit to be easily realized by an integrated circuit (refer to, for example, Japanese Laid-Open Patent Publication No. 7-182055). The circuit described in this publication is shown in correspondence with the present invention in FIG. 4. The stabilizing power supply circuit includes an output transistor and a current detection resistor, which are connected between an input terminal and an output terminal. The stabilizing power supply circuit further includes an error amplifier and a first output control transistor for comparing the voltage of the output terminal with a reference voltage and controlling the output transistor. The stabilizing power supply circuit further includes a comparator and a second output control transistor for detecting the voltages at the two terminals of the current detection resistor and controlling the output transistor. In this way, the stabilizing power supply circuit separately sets the overcurrent detection point and the output current when a ground fault occurs in the output terminal. This reduces the power consumed by the output transistor when a ground fault occurs in the output terminal and prevents heat generation or breakage of the circuit, which would be caused by a fault occurring in the output terminal.
A technique relating to an overcurrent protection circuit that does not have a current detection resistor on its output path has been proposed (refer to, for example, Japanese Laid-Open Patent Publication No. 11-103524). The circuit described in this publication is shown in correspondence with the present invention in FIG. 5. This overcurrent protection circuit includes an output transistor for outputting current supplied to a load, a drive transistor for controlling the amount of current flowing through the output transistor, and a feedback means for controlling the drive transistor in accordance with the value of output voltage supplied to the load. The overcurrent protection circuit detects current corresponding to the current flowing through the load. When the detected current is greater than or equal to a predetermined value, the overcurrent protection circuit controls the drive transistor to limit the output current of the output transistor. In this way, the overcurrent protection circuit directly controls the current of the drive transistor in accordance with the detected current value. This configuration enables the overcurrent protection function to be realized using an independent reference voltage. Thus, the overcurrent protection function is less dependent on the voltage of the transistor, the input voltage, and the output voltage. Further, the overcurrent protection circuit does not required a current detection resistor to be arranged in its current path that leads to the output terminal, and a current mirror circuit may be arranged in a path through which current proportional to the output current flows.
A technique relating to a constant voltage power supply circuit, which enable its overcurrent limiting function with respect to overload and its short-circuit current limiting function with respect to output short-circuiting to be set freely in a manner independent of each other, has been proposed (refer to, for example, Japanese Laid-Open Patent Publication No. 2002-169618). The circuit described in this publication is shown in correspondence with the present invention in FIG. 6. In the constant voltage power supply circuit, an output transistor is controlled based on the output of an error amplifier. Load current is detected by a current detection circuit. Voltage generated based on the detected load current and the output of a voltage detection circuit detecting output voltage are input into a comparator having an offset. Then, the output transistor is controlled based on the output of the comparator.
In the circuit according to the technique described in Japanese Laid-Open Patent Publication No. 7-182055, the resistor, which detects the current, consumes much power. If current is decreased, detection would become difficult. When detecting current with a resistor, the resistance would have to be accurate. However, it is difficult to manufacture a small resistor having a precise absolute value during a semiconductor fabrication process. From this viewpoint, it would be difficult for the circuit described in Japanese Laid-Open Patent Publication No. 7-182055 to be realized by an integrated circuit.
In the circuit described in Japanese Laid-Open Patent Publication No. 11-103524, it may be difficult to detect short-circuit current depending on the voltage condition or changes in the environment in which the detection transistor. Further, the value of the short-circuit current, which is determined using the absolute value of the resistor, may greatly differ depending on the resistance of the resistor.
Likewise, in the circuit in Japanese Laid-Open Patent Publication No. 2002-169618, it may be difficult to detect short-circuit current depending on the voltage condition or changes in the environment in which the detection transistor. Further, the value of the short-circuit current changes when the threshold voltage of the transistor and the resistance of the resistor change.